1. Field of the Invention
The present invention relates to a transmission input/output device for a signal transmission between transmission terminals through a telephone line either in a state of picking up the telephone line or not picking up the telephone line.
2. Description of the Background Art
An example of a conventional transmission input/output device is shown in FIG. 1, where terminals T1 and T2 are connected to the telephone line, and a primary coil of a transformer 51 and a resistor 53 are connected in series between the terminals T1 and T2, while a secondary coil of the transformer 51 is connected to a transmitter circuit 59 through a resistor 55 and an amplifier 57 as well as to a receiver circuit 63 through the resistor 55 and a differential amplifier 61.
The amplifier 57 comprises: an operational amplifier 65; a resistor 67 connected to an input line between the transmitter circuit 59 and an inverting input terminal of the operational amplifier 65; and a resistor 69 connected to a feedback line between the inverting input terminal and an output terminal of the operational amplifier 65.
The differential amplifier 61 comprises: an operational amplifier 77; a resistor 71 connected to an input line between an inverting input terminal of the operational amplifier 77 and the resistor 55; a resistor 73 connected to another input line between the transformer 51 and a non-inverting input terminal of the operational amplifier 77; a resistor 75 connected to a feedback line between the inverting input terminal and an output terminal of the operational amplifier 77; and a resistor 79 connected to the non-inverting input terminal of the operational amplifier 77.
The transformer 51 possesses a high impedance of approximately 600 D with respect to the alternating current, and a low impedance with respect to the direct current. The resistor 53 connected in series with the primary coil of the transformer 51 determines the direct current impedance for the telephone line. The resistor 55 connected to the secondary coil side of the transformer 51 is selected to be 600 .OMEGA., for example, in order to take an impedance matching in the case of transmission signal reception.
In such a conventional transmission input/output device, the transmission signals to be transmitted through the telephone line are outputted from the transmitter circuit 59, amplified by the amplifier 57, and transmitted onto the telephone line through the transformer 51, while the transmission signals to be received through the telephone line are supplied to the differential amplifier 61 through the transformer 51, amplified by the differential amplifier 61, and received by the receiver circuit 63.
Such a conventional transmission input/output device has been associated with problems that it has been difficult to reduce its size and cost because the transformer 51 is involved, and that the power source such as batteries of the transmission terminal circuit in which the transmission input/output device is incorporated have to be large because the current consumption by the amplifier 57 and the differential amplifier 61 is so large that the satisfactory operation may be hampered by the lack of amplitude of the transimission signals in a case where the amplifier 57 and the differential amplifier 61 are operated at a low voltage of 3 or 5 V, as a consequence of the loss at the transformer 51.